Composite having transparent conductor pattern

ABSTRACT

A continuous layer of transparent material is formed on a base. A desired pattern of transparent electrically conductive material is included in the layer. The regions of the layer not included in the conductor pattern are insulative. The insulative material is preferably an indium oxide. Tin is preferably used as a dopant in the indium oxide to made it electrically conductive according to the desired pattern. The electrically conductive pattern is first formed in metallic tin. The tin pattern is diffused into a layer of metallic indium. The layer is then thermally oxidized to form the electrically conductive pattern as indium-tin-oxide (ITO) in indium sesquioxide, In 2  O 3 , an insulator.

This is a continuation, of application Ser. No. 925,245 filed Jul. 17,1978, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the invention

This invention relates to transparent electrically conductive materialsystems.

2. Description of the Prior Art

Layers, coatings, and thin films of metal oxides which are electricallyconductive are well known. These have been used extensively in thefabrication of, for example, display devices such as liquid-crystaldisplays wherein some of the electrodes are required to be transparentso as not to impair the viewability by an observer of the displayedinformation. For an example of this application, see Schindler, U.S.Pat. No. 3,814,501. The most frequently encountered metal oxides usedfor this purpose are oxides of indium, tin, and of indium and tintogether (indium-tin-oxide or ITO). Schindler discloses patterns oftransparent, electrically conductive material in a liquid-crystaldisplay where the conductive material is indium oxide or tin oxide or acombination of these materials.

Another application for transparent electrical conductors is aselectrodes for gate structures which are fabricated on suchsemiconductor devices as charge-coupled imagers.

In both the imager device and the display device applications, thepractice has been to form the conductor pattern as a separate,discontinuous entity. Typically, the pattern has been formed bydepositing metallic oxide material and then using photolithographictechniques to conform the metallic oxides to a desired conductor patternalthough various other approaches such as, for example, printing havealso been used. For an example of the latter, see Laurie, (West German)Offenlegungsschrift No. 2,411,872.

Where a conductor pattern of, for example, ITO is formed as a separateentity on an imager or display device, at least two distinct types ofpaths are created for light which is incident on the device. In one typeof path, a light beam passes through the thickness of the ITO once ortwice depending on whether or not it is reflected. In the other type ofpath, which does not pass through an ITO layer, the light typicallytravels a longer distance through air-filled space. The differencebetween the indexes of refraction of the air and ITO and the differencein the number of interfaces traversed by a light beam depending on thepath traveled produces distortion in the image sensed or reflected. Thesubject invention greatly reduces these differences thereby reducing theresultant distortion.

Barrett et al, U.S. Pat. No. 2,932,590, discloses the controlleddeposition of metallic indium and tin and the subsequent thermaloxidation thereof to produce thin, transparent, electrically conductivecoatings. However, Barrett et al does not disclose the formation of apattern of electrical conductors included in an insulative film asdisclosed herein.

SUMMARY OF THE INVENTION

The present invention provides a pattern of transparent electricallyconductive material deposited on a substrate, a semiconductor device, orthe like, wherein the pattern of electrically conductive material isincluded in an otherwise continuous layer of transparent insulativematerial having substantially the same index of refraction as theelectrically conductive material, the layer having substantially thesame thickness throughout. Similar optical paths are provided for lightpassing through conductor material and for light passing throughinsulator material because length and propagation time differences forlight rays passing through the two different types of material are madesmall.

In the preferred embodiment, the insulator material is an indium oxide,more particularly indium sesquioxide (In₂ O₃). In order to make theratio of the resistivity of the insulator material to the resistivity ofthe conductor material high, the insulative indium oxide is preferablymade to be relatively free of impurities and substantiallystoichiometric. Impurities and deficiencies of oxygen ions provide freecharge carriers and are undesirable for that reason.

Regions of the layer of insulator material are modified to make themelectrically conductive in a pattern conforming to the desired conductorpattern. In the preferred embodiment, tin is used as a dopant to modifythe insulator material.

Both the tin and indium are initially deposited on the substrate ordevice in elemental, or metallic, form. The tin is conformed to thedesired pattern for the electrical conductors. The tin is then diffused,or alloyed, into the indium and the material system is oxidized.

Elemental materials are obtainable in significantly higher purities thanare, for example, metallic oxides. By working with initially metallictin and indium then, the purities of the materials used in practicingthis invention can be made consistent with the requirements ofsemiconductor technology wherein even small amounts of contaminants areknown to cause degradation of device parameters.

As another advantage of the subject invention, it will become apparentthat smaller electrical conductor linewidths can be implemented in thesubject invention than have been conveniently possible to implement inthe prior art. That is because the conductor pattern is first formed ina thin layer of modifying material, such as the tin mentioned above,which is significantly thinner than the finally formed conductors. Thinlayers can be etched to smaller linewidths because the reduced timerequired for etching greatly reduces undercut.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E are schematic diagrams showing, in sectional views, stagesin the formation of conductors according to a preferred embodiment ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1A, a base 10 is shown on which there isdeposited a layer 12 of a modifying material. The base 10 may be, forexample, a glass substrate used as a cover plate for a liquid-crystaldisplay device wherein transparent electrodes are desired. As anotherexample, the base 10 may be a semiconductor integrated circuit devicesuch as a CCD imaging device wherein transparent conductors are used inthe gate structure. In the preferred embodiment, the modifying materialdeposited is metallic tin. High purity tin may be preferred for use incases where the base is, for example, a semiconductor device, asmentioned above, and where properties of the semiconductor device may beadversely affected by impurites in the tin. Metallic tin having a purityas high as 99.999% may be readily obtained and used in practicing thisinvention.

In the preferred embodiment, the metallic tin is preferably depositedusing standard sputtering techniques well known to those skilled in theart. A suitable power level for rf sputtering of tin is about 150 watts.A suitable gas pressure for the argon contained in the sputteringchamber is 8±2 microns.

Instead of sputtering, however, any of the various other standardtechniques for the deposition of metals may be used, if so desired, forthe deposition of the layer 12 of tin. These other techniques include,for example, plating, plasma coating, vacuum vapor deposition, pyrolyticcoating, thermal evaporation, and electron-beam evaporation.

A preferred thickness for a completed layer of transparent materialformed in accordance with this invention is about 1000 angstroms. Thisthickness represents a compromise providing adequately high conductivityfor the conductive regions of the completed layer and adequately highresistivity for the insulative regions of the completed layer inelectronic applications, such as in display or imaging devices, alongwith high light transmittance. For a completed layer thickness of 1000angstroms, the preferred thickness for the deposited layer 12 of tin is75±25 angstroms. Thus, it is seen that in the preferred embodiment thelayer 12 of modifying material has a thickness only 10%, or less, or thethickness of the completed layer.

After the layer 12 of modifying material is deposited on the base 10 asshown in FIG. 1A, the layer 12 is formed, or delineated, into a patternconforming to a desired pattern for electrical conductors. The result isillustrated in FIG. 1B wherein individual elements 14 of modifyingmaterial are shown disposed on the base 10 and positioned thereon in thedesired locations for conductors. A plurality of individual elements 14are shown in FIG. 1B for the sake of illustration only. It will berecognized by those skilled in the art that the subject invention isequally applicable to cases where a desired conductor pattern has but asingle unitary conductor structure.

Where the modifying material is tin, the individual elements 14 arepreferably conformed to the desired conductor pattern by standardphotolithography techniques well known to those skilled in the art. Forexample, a suitable photoresist for this purpose is manufactured andsold under the trademark Kodak #809 by the Eastman Kodak Company ofRochester, New York. The photoresist is preferably baked at a relativelylow temperature, for example no higher than about 90° C., to preventdiffusion of the tin into the photoresist. A suitable etchant for thetin is a mixture of 12.5 grams of ammonium fluoroborate, 2.25 liters ofconcentrated nitric acid, 0.2 liters of concentrated fluoroboric acidand 150 liters of deionized water. Using this etchant, the elements 14can be formed in the tin in about 20 seconds.

It is well known that patterns delineated by photolithographictechniques are limited in their definition by reason of the undercutproduced by etchants. However, in the method of forming the desiredelectrical conductor pattern described thus far, the pattern is formedin a layer 12 having a thickness 10% or less, of the thickness of thecompleted layer of transparent material in which the pattern forelectrical conductors will ultimately be included, as discussedhereinbelow. It is clear then that a conductor pattern having finerdefinition (smaller linewidths and smaller line-to-line spacing) can bephotolithographically produced in conjunction with the practice of thisinvention than can be produced in the case where a pattern is requiredto be formed in a layer having the greater thickness of the completedlayer of transparent material. The latter case represents the situationin the prior art where layers of conductive metallic oxides are etchedto form the desired pattern of electrical conductors.

Although photolithographic techniques are preferred for forming theelements 14 into the desired conductor pattern, other approaches may beused consistent with the invention. For one example, the modifyingmaterial may be deposited through a mask thus forming the elements 14 ofFIG. 1B immediately. This approach may be more desirable where theelements 14 are to be established on a base having sharp corners orsteps to which the elements 14 are required to conform. As yet anotherexample, the modifying material may be disposed in an ink which isscreen printed onto the base in the desired pattern. Using either ofthese two techniques for forming the elements 14 may have the result ofreducing the number of steps required to form a completed layer inasmuchas an undelineated layer of modifying material such as the layer 12 ofFIG. 1A is not required to be deposited.

As shown in FIG. 1C, the elements 14 of modifying material, after beingformed, are covered with a layer 16 of a material which is bothoxidizable into a transparent insulative material when relatively pureand oxidizable into a transparent conductive material when combined witha suitable modifying material. Metallic indium having a purity of atleast about 99.9% by weight, is the preferred oxidizable material foruse with tin in practicing this invention. As with the tin used to formthe layer 12 of modifying material in FIG. 1A, a greater purity for theindium may be preferred for use in cases where the base is, for example,a semiconductor device and where properties of the semiconductor devicemay be adversely affected by impurites in the indium. Metallic indiumhaving a purity as high as 99.999% by weight may be readily obtained andused in practicing this invention.

An additional reason exists for using a relatively higher purity indium.Oxidized indium is to serve as an insulator in a completed layer oftransparent material conforming to the invention. Impurities in theindium will tend to provide free charge carriers and thereby decreasethe electrical resistivity of the indium oxide. The extent to whichdecreased resistivity of the insulative indium oxide due to impuritiesis a problem depends, in part, on the application for which theinvention is practiced. Where the completed material system isrelatively large, where the desired pattern for the conductive regionsthereof provides relatively large conductors spaced relatively farapart, satisfactory performance may be obtainable with a relatively lowratio of the resistivity of the insulative material to the resistivityof the conductive material. On the other hand, for applicationsinvolving microelectronic circuitry and highly precise definition ofsmall conductors, a ratio of the resistivity of the insulative materialto the resistivity of the conductive material as high as it ispracticable to obtain is preferred. For the latter situation, thegreater the purity of the metallic indium used, the greater the ratio ofresistivities will tend to be. A ratio of resistivity of insulativematerial to resistivity of conductive material as high as 10⁶ has beenobtained in practicing the subject invention using indium having apurity of about 99.999% by weight.

In the preferred embodiment, the metallic idium is preferably depositedusing standard sputtering techniques well known to those skilled in theart. A suitable power level of rf sputtering of indium is about 175±25watts. A suitable gas pressure for the argon contained in the sputteringchamber is 8±2 microns.

Instead of sputtering, however, any of the various other standardtechniques for the deposition of metals may be used, if desired, for thedeposition of the layer 16 of indium. As was mentioned above inconnection with the discussion of techniques for depositing tin, thoseother techniques include, for example, plating, plasma coating, vacuumvapor deposition, pyrolytic coating, thermal evaporation, andelection-beam evaporation.

For that preferred embodiment wherein the completed layer of transparentmaterial has a thickness of about 1000 angstroms and wherein the layer12 of tin, as shown in FIG. 1A, is 75±25 angstroms thick, the preferredthickness for the layer 16 of indium is about 700 angstoms.

The layer 16 of indium must cover the entire area on the base 10 forwhich it is desired to have a coating of transparent material. Inasmuchas the deposition of the layer 16 of indium may have covered portions ofthe base 10 over which no coating of transparent material is desired,photolithographic forming techniques may be used to remove the indiumfrom those portions. Such would be the case, for example, where accessto pads on an underlying semiconductor device was required to be madeavailable for connection to interface circuitry not provided by theprocedures of this invention. In this event, the layer 16 of indium ispreferably formed using the same photoresist and the same etchantdescribed above in connection with the discussion of forming theelements 14 as shown in FIG. 1B. Approximately two minutes is requiredto etch this indium layer 16 using the aforementioned etchant.

After the elements 14 of modifying material are covered with a coatingof oxidizable material 16 as shown in FIG. 1C, the modifying material isdiffused into the oxidizable material. This may be accomplished in anoven having either an inert or a reducing atmosphere. Where thematerials are preferred dopant, tin, and the preferred oxidizablematerial, indium, the diffusion, or alloying, is preferably accomplishedby heating in a reducing atmosphere of about 85% nitrogen and 15%hydrogen.

FIG. 1D illustrates the result of the diffusion of the tin into theindium in regions 18 throughout the depth of the layer 20. The regions18 of the layer 20 conform to the desired pattern for electricalconductors while the remaining portions of the layer 20 are intended tobe occupied by insulative material.

The definition of the conductor pattern to which the regions 18 ofalloyed material can be required to conform is limited by a need tomaintain a certain minimum spacing for insulation between theconductors. The spacing obtained between regions 18 of alloyed materialis dependent both upon the spacing provided between the elements 14 ofmodifying material, as shown in FIGS. 1B and 1C, and upon the manner inwhich the diffusion of the modifying material into the oxidizablematerial is carried out. The spacing between the adjacent edges of theelements 14 of modifying material, as shown in FIGS. 1B and 1C, ispreferably at least about ten times the thickness to which the layer 16of oxidizable material is deposited. For a layer 16 having a thicknessof, for example, 700 angstroms, the preferred spacing between theelements 14 is preferably at least 7000 angstroms, or about one micron.This spacing lower limit is consistent with the minimum line-to-linespacing which is reasonably obtainable for a 700 angstrom layer byphotolithographic techniques.

The extent to which the modifying material is diffused into theoxidizable material is determined, in part, by the parameters of theheating operation used to produce the diffusion. In the preferredembodiment, the alloying of the tin and indium is accomplished at atemperature of 200°±20° C. for 30±10 minutes. The diffusion must belimited since heating for too long a time or at too high a temperature,or both of these, will ultimately result either in conductors having toolittle spacing between them or, eventually, in the modifying materialbeing uniformly distributed throughout the entire layer 20 of FIG. 1D.In either case, the desired conductor pattern would be effectivelyeliminated. Heating at lower temperatures or for shorter times, or bothof these, may prevent adequately good mixing of the indium and tin inthe regions 18 of the layer 20.

It will be apparent to those skilled in the art that the proceduralorder described hereinabove for producing the material systemillustrated in FIG. 1D, while it is preferred, is not exclusive. Forexample, the layer 16 of indium may be deposited first, before the layer12 of tin, directly on the base 10. After the layer 16 of indium isformed, the tin layer 12 may be deposited and photolithographicallyformed on top of the indium layer 16 using well known lift-offtechniques and employing a photoresist mask to protect the indium frometchant. Once the tin is diffused into the indium, the result will be asis illustrated in FIG. 1D.

The structure illustrated in FIG. 1D is converted to the one shown inFIG. 1E by heating the layer 20 in oxygen or air to oxidize it. Theunmodified oxidizable material is converted to transparent insulativematerial in the completed layer 22 of FIG. 1E. The modified material ofthe regions 18 of FIG. 1D is converted to transparent electricallyconductive material in the regions 24 of FIG. 1E.

Where the oxidizable material is the preferred metal indium, thetransparent insulative material formed is an indium oxide. Completeoxidation to produce stoichiometric indium sesquioxide (In₂ O₃) isbelieved to be preferable for the higher resistivity obtainable. Wherethe modifying material is tin, the indium-tin alloy is converted toindium-tin-oxide (ITO) a well known transparent electrical conductorwherein the tin acts as a dopant for the otherwise insulative indiumoxide.

Stoichiometry of the indium sesquioxide is believed to be assured byheating for a sufficient time at a sufficiently high temperature.Heating in air at 480°±20° C. for from 30 to 120 minutes appears to givesatisfactory results.

The transparent layer 22 of metallic oxide material grows in thicknesswhile being oxidized. For a metallic layer originally about 700-800angstroms thick, the oxide layer 22 of FIG. 1E will be about 1000angstroms thick. The film growth from metal to metallic oxide is by afactor of from about 1.35 to about 1.45.

It is a goal to control the thickness of the indium and tin layers inorder to achieve, after processing, a composition of the ITO which isapproximately 90 mole percent of indium oxide and 10 mole percent of tinoxide. This is the commonly accepted composition of the most conductiveITO.

Typical measured values obtained for the refractive index of the indiumoxide have been in the range of from 1.7 to 1.8. Measured value for therefractive index of the ITO have been in the range from 1.9 to 2.0.Measured values for the resistivity of the indium oxide have ranged from1000 ohm-cm to 10,000 ohm-cm while the resistivity of ITO produced bythe subject invention has ranged from 0.01 ohm-cm to 0.02 ohm-cm.

While the invention has been described in its preferred embodiment, itis understood that the words which have been used are words ofdescription rather than of limitation and that changes within thepurview of the appended claims may be made without departing from thetrue scope and spirit of the invention in its broader aspects. Forexample, tin, when relatively pure, may be used as the materialoxidizable to form transparent insulator material. Antimony may be usedas a dopant to modify the tin oxide to be conductor material. As anotherexample, cadmium as the modifying material may be diffused into tinserving as the oxidizable material. Oxidation of the cadmium-tin alloywill produce the compound cadmium stannate (Cd₂ SnO₄) which is known tobe highly conductive. Furthermore, those skilled in the art willrecognize that the subject invention is not limited to a single layer oftransparent material which includes regions of conductive material. Aplurality of such layers may be formed in a stack if so desired. It maybe desirable to separate the layers which include conductor material byother layers of insulator material only.

What is claimed is:
 1. A composite comprising:an integrated circuitdevice of semiconductor material, and a substantially continuous layerof transparent material disposed on said integrated circuit device, saidlayer of transparent material having a first region therein ofinsulative material and a second region therein of electricallyconductive material,wherein said second region is determined bydiffusing a modifying material into a portion of a layer of a firstmetal disposed on said base whereby said first metal is alloyed withsaid modifying material; wherein said insulative material is an oxide ofsaid first metal formed by oxidation of unalloyed parts of said layer ofa first metal, wherein said electrically conductive material is anoxidation product of the alloyed portion of said layer of a first metal,wherein said first metal in said layer has a purity of at least about99.9% by weight, and wherein said insulative material is a substantiallypure and stoichiometric oxide of said first metal having a resistivityof at least about 1000 ohm-cm.
 2. A composite as recited in claim 1wherein the ratio of the resistivity of said insulative material to theresistivity of said electrically conductive material is at least about50,000.
 3. A composite as recited in claim 2 wherein said first metal isindium and wherein said modifying material is tin.
 4. A composite asrecited in claim 2 wherein said first metal is tin and said modifyingmaterial is antimony.
 5. A composite as recited in claim 2 wherein saidelectrically conductive material is a compound of said first metal andsaid modifying material and wherein said compound is formed by oxidationof an alloy of said first metal with said modifying material.
 6. Acomposite as recited in claim 5 wherein said first metal is tin and saidmodifying material is cadmium.